Method of strengthening Ti alloy

ABSTRACT

Ti alloy is heated in an atmosphere of CO 2  in a heating furnace. O and C atoms are introduced into the Ti alloy to harden it without forming Ti oxide, thereby increasing hardness by Ti—O and Ti—C solid solutions thus formed.

BACKGROUND OF THE INVENTION

[0001] This invention relates to a method of strengthening Ti alloy toimprove wear resistance.

[0002] In automobile industries, poppet valves and other valve-operatingparts are made of Ti alloy that provides high strength and low specificgravity. Poppet valves require wear resistance and scuff resistance atportion which is engaged with different valve-operating parts.

[0003] In order to strengthen Ti alloy material to provide wearresistance and scuff resistance, various methods have been developed.For example, oxides are formed on the surface of Ti alloy in JapanesePatent Pub. No. 62-256956. Nitrides are formed on the surface inJapanese Patent Pub. No. 61-81505. Carburizing is carried out to diffusecarbon atoms into Ti alloy in Japanese Patent No. 2,909,361.

[0004] Wear resistance and scuff resistance in the foregoing methods areimproved in Ti alloy material, but the surface is so hard that differentparts to be engaged are likely to be attacked.

[0005] Japanese Patent Application No. 2001-25415 discloses a Ti alloypoppet valve in which Ti—O and Ti—C solid solutions are formed, and amethod of manufacturing a Ti alloy poppet valve, comprising the steps ofheating the Ti alloy valve at temperature lower than beta transformationpoint in a plasma vacuum finance which contains oxygen less thanstoichiometric amount for forming Ti oxides to diffuse O and C atoms toform O and C diffusion layer which comprises Ti—O and Ti—C solidsolutions to strengthen a valve body.

[0006] To diffuse O and C atoms, in the presence of O₂ less thanstoichiometric amount for forming titanium oxides, heat treatment iscarried out at about 800° C. Glow discharge is made in the presence of agas for ionized carburizing, or plasma carburizing is carried out whileoxygen less than stoichiometric amount for forming titanium oxide issupplied. Oxygen/carbon diffusion layer thus obtained not only improveswear and scuff resistance, but also decreases attacking property toother members.

[0007] However, as mentioned above, heat treatment is carried out in thepresence of oxygen in a plasma vacuum finance and ionizing carburizingis carried out by glow discharge, which is complicate. Furthermore, itis necessary to employ a vacuum discharge device and plasma power sourcein a plasma vacuum finance to increase cost.

SUMMARY OF THE INVENTION

[0008] In view of the disadvantages in the prior art, it is an object ofthe present invention to provide a method of strengthening Ti alloy todiffuse oxygen and carbon atoms without forming titanium oxide.

[0009] According to the present invention, there is provided a method ofstrengthening Ti alloy, comprising the step of heating the Ti alloy inan atmosphere of CO₂ at 600 to 900° C. in a heating furnace to diffuse Cand O atoms into the Ti alloy.

BRIEF DESCRIPTION OF THE DRAWINGS

[0010] The features and advantages of the present invention will becomemore apparent from the following description with respect to appendeddrawings wherein:

[0011]FIG. 1 is a micrograph of Ti alloy treated by Example 1 of thepresent invention;

[0012]FIG. 2 is a graph that shows oxygen and carbon atom concentrationof the Ti alloy material in FIG. 1;

[0013]FIG. 3 is a graph of hardness to depth of Ti alloy material inFIG. 1;

[0014]FIG. 4 is a micrograph of Ti alloy treated in Example 3 of thepresent invention;

[0015]FIG. 5 is a micrograph of Ti alloy treated in Comparative Example2;

[0016]FIG. 6 is a graph of the results of wear test to Ti alloymaterials; and

[0017]FIG. 7 is a schematic view of a device for the wear test.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

[0018] Heat treatment of the present invention will be described asbelow.

[0019] Ti alloys include alpha alloys such as Ti—5Al—2.5Sn; near-alphaalloys such as Ti—6Al—2Sn—4Zr—2Mo(hereinafter refer to “Ti6242”) andTi—8Al—Mo—V; alpha-beta alloys such as Ti—6Al—4V, Ti—6Al—6V—2Sn andTi—6Al—2Sn—4Zr—6Mo; and beta alloys such as Ti—13V—11Cr—3Al andTi—15Mo—5Zr—3Al. Ti6242 may be preferably used.

[0020] In heat treatment, Ti alloy is put in a heating furnace, and airin the furnace is purged by CO₂. It is heated in an atmosphere of CO₂ at600 to 900° C., preferably 800 to 850° C.

[0021] Below 600° C., diffusion speed of carbon atoms is too slow, whichis disadvantageous in cost. Above 900° C., oxide layer is formed and thetemperature exceeds beta transformation point of Ti to change itsconfiguration, which is not preferable.

[0022] In heat treatment, to supplement CO₂ consumed by introduction ofC and O into Ti alloy and to maintain CO₂ atmosphere in the furnace, CO₂may be always fed into the heating furnace. Feeding rate may be 0.5 to3.0 l/min, preferably 1.0 to 2.5 l/min.

[0023] Time for treatment in CO₂ affects wear resistance or hardness,and may be preferably 1 to 3 hours. By the heat treatment, O and C atomsare diffused at depth of 25 to 50 μm from the surface, and surfacehardness is HV 550 to 1000.

[0024] When a poppet valve in an internal combustion engine of anautomobile is made of Ti alloy, suitable Vickers hardness is HV 700 to850. The valve treated by the method of the present invention not onlyprovides wear and scuff resistance, but also improves attacking propertyto the other member.

EXAMPLE 1

[0025] In a muffle furnace which has volume of 24 l, a poppet valve madeof Ti6242 was put as sample and CO₂ was introduced to purge air. CO₂ wasfed into the furnace at the flow rate of 1 l/min and the sample washeated till 800° C. and maintained at the temperature for two hours.Then, the valve was cooled to room temperature without contacting air.After cooling, the sample was taken out of the furnace and various testswere carried out.

[0026]FIG. 1 illustrates a micrograph of a section of the sample. Asillustrated in the micrograph, O and C atoms were introduced at thedepth.

[0027]FIG. 2 is a graph which shows averages of concentrations of O andC atoms measured at each depth by an electric-field-radiation-type Augerelectronic spectrometer. In the graph, an axis of abscissa denotes depth(μm) from the surface of the sample, and an axis of ordinate denotesconcentration (atomic %) of O and C atoms. The unit of concentration“atomic %” means rate of O and C atoms with respect to analyzed totalatoms. The graph shows oxygen and carbon atoms in the diffusion layer ofthe sample.

[0028] X-ray diffraction in X-ray microdiffraction device identifiesTiC, but does not find titanium oxide. From the result, oxygen atoms donot combine with titanium, but remain as atoms. Carbon atoms partiallycombine with titanium to form TiC, but the remaining is diffused ascarbon atoms.

[0029] Section hardness of the sample thus obtained was measured by aMicro-Vickers hardness tester of Shimazu Corp. FIG. 3 shows distributionof hardness. An axis of abscissa means depth (μm) from the surface, andan axis of ordinate means hardness (HV) under 100 gf. It showsimprovement in hardness up to depth of 50 μm according to the method ofthe present invention.

[0030]FIGS. 2 and 3 prove that existence of oxygen and carbon atomscontributes improvement in hardness of Ti alloy.

[0031] As shown in FIG. 3, surface hardness was HV 830.

EXAMPLES 2 TO 9 AND COMPARATIVE EXAMPLES 1 to 3

[0032] Surface treatment was carried out under different temperaturesand time with respect to Ti6242, and the following Table shows theresults. TABLE Surface Temperature Time Hardness Oxide (° C.) (h) (HV)Layer Example 2 750 3 570 none 3 800 0.5 630 none 4 710 50 680 none 5800 1 710 none 6 800 1.5 790 none 7 800 3 870 none 8 850 1 930 none 9850 2 960 none Comparative 850 55 1030 formed Example 1 2 900 1 980formed 3 1000 0.5 1030 formed

[0033] In an atmosphere of CO₂, Ti6242 was heated at 710 to 850° C. for0.5 to 50 hours, so that O and C atoms were introduced into Ti alloywithout forming oxide.

[0034]FIG. 4 illustrates a microgragh of a Ti alloy poppet valve treatedIn the Example 3, and O and C diffusion layer was formed.

[0035] A poppet valve is used in an internal combustion engine of anautomobile and is subjected to severe condition such as hightemperature. Such a valve requires hardness of HV 700 to 850. InExamples 1, 5 and 6, a sample requires to be subject to the conditionsof time for 1 to 2 hours at 800° C.

[0036] As clarified in Comparative Example 1, the temperature 850° C.was the same as those in Examples 8 and 9, but it took 55 hours toattain HV 1030. But it was so long that an oxide layer was formed on thesurface. Deformation is large and it is not suitable.

[0037] In Comparative Examples 2 and 3, when the temperature was over900° C., surface hardness was sufficient, but a thick oxide layer wasformed to cause large deformation, which was not suitable for actualuse.

[0038]FIG. 5 shows a micrograph of a poppet valve in Comparative Example2, in which an oxide layer was formed on an O and C diffusion layer.

[0039]FIG. 6 illustrates results of wear tests of Ti6242 in Examples 1and 3, Comparative Example 2, untreated Ti alloy and tuftriding-treatedheat-resistant steel.

[0040] To carry out the test, as shown in FIG. 7, a test piece 2 isengaged in a valve guide 1 made of Fe-sintered material. Vertical weight“W” for 6 kgf was loaded and the test piece 2 was reciprocally slid for50 hours while lubricating oil was supplied between them.

[0041] The test piece made of untreated Ti6242 was the maximum in wear,and wear becomes smaller in order of Example 3, Example 1,heat-resistant steel and Comparative Example 2. Example 1 is equivalentto the heat-resistant steel in wear. Owing to difference in surfacehardness, Example 3 is larger than Example 1 in wear. The minimum wearin Comparative Example 2 seems to be due to an oxide layer on thesurface. Comparative Example 2 was too rigid, so that wear of the valveguide 1 engaged therewith was the maximum.

What is claimed is:
 1. A method of strengthening Ti alloy, comprisingthe step of: heating the Ti alloy in an atmosphere of CO₂ at 600 to 900°C. in a heating furnace to diffuse C and O atoms into the Ti alloy.
 2. Amethod as claimed in claim 1 wherein the method is carried out for 0.5to 50 hours.
 3. A method as claimed in claim 1 wherein the method iscarried out at 800 to 850° C.
 4. A method as claimed in claim 3 whereinthe method is carried out for 1 to 3 hours.
 5. A method as claimed inclaim 1 wherein CO₂ is always introduced into the heating furnace.
 6. Amethod as claimed in claim 1 wherein the method is carried out at about800° C. for 1 to 2 hours.
 7. A method as claimed in claim 6 wherein theTi alloy is used to make a poppet valve in an internal combustionengine.